Adhesive tape



Patented Oct. 23, 1951 ADHESIVE TAPE Warner Eustis, Newton, Mass, assignor to The Kendall Company, Boston, Mass., a corporation of Massachusetts No Drawing. Original application May 18, 1944,

Serial No. 536,198. Divided and this application March 7, 1951, Serial No. 214,443

Claims. 1

This invention relates to pressure-sensitive adhesive tapes and is concerned particularly with the formulation of the adhesives thereof. Adhesives formulated in accordance with this invention, especially when combined with suitable and preferred types of backings as hereinafter described, provide pressure-sensitive adhesive tapes having highly improved electrical properties, including low power factor and. high dielectric strength, while. combining" with these special qualities commercially satisfactory characteristics of adhesive. pressure-sensitivity, ad:- hesion, and ageing (in some cases by the utilization of prepared containers), so that the tape .may be preserved over substantial periods of time.

with its adhesive remaining in a soft, viscous, tacky, pressure-sensitive condition, whereby the tape may be manipulated and applied to an external surface in the ordinary manner of maskingor of surgeons" tape.

In order to impart satisfactory pressure-sensitive characteristics to an adhesive and, particularly, good adhesion characteristics, rosins and/or resins with or without non-volatile liquid plasticizers are widely used as necessary modifiers for the film-forming or elastomeric ingredi ent of the adhesive, which ingredient, being chosen for its elastic and tensile strength qualities, usually has insufiicient adhesion without modification. The dielectric properties of these rosins, resins and plasticizers are in general so inferior and affect the insulating qualities of adhesives containing them so adversely that presentday pressure-sensitive adhesive tapes do not find as great a use in the electrical industry as their convenience Would appear at first glanceto warrant.

It is an object of this invention to provide adhesives, having commercially satisfactory and preferably stable qualities of tack and adhesion, which help to give a tape useful electrical properties such as high breakdown voltage and low power factor.

Adhesives of this invention are structurally novel because, as distinguished from compounded adhesives having high molecular weight elastomers, for instance natural rubber, which are modified in the direction of increased adhesion and/or reduced viscosity by the inclusion or compatible but molecularly foreign modifiers. (i. e.-

rosin, resin, and non-volatile liquid plasticizers),

they consist, in the preferred form, solely of polymolecular polymeric systems-or such systems containing. additional solids, which are inert, i. e. fillers-wherein the. molecular weight. distribution of the polymers in the system. (being preferably all polymers of the same monomeric unit) is a predeterminedly controlled perturbation of a normal polymolecular distribution, presenting an abnormal or unnatural distribution of molecular Weights. By such deliberate perturbation, elasto-viscous polymeric materials are obtained which per se, or in the presence of the inert filler, possess the required viscous, tacky, and preferably stable pressure-sensitive adhesive characteristics, and which are spreadable into coherent sheets as by a solvent spreading or calendering operation, in forms useful as pressure-sensitive adhesive coats on suitable backings, to form pressure-sensitive adhesive tapes.

High molecular Weight polymeric substances, both natural, such as polyisoprene, and synthetic, such as high molecular weight polybutadiene, polybutene, and polystrene, and copolymers thereof are known to consist of molecules of widely different size. The molecular weights of the different molecules are distributed about some average value, and in the normal aggregation, there are molecules much smaller as well as much larger than this average value.

The normal distribution of molecular Weight species produced in a normal linear polymerization reaction is known. According to Flory (Journal American Chem. Society, 58, 1877 (1936)), as the reaction approaches completion the number of polymers in any single molecular species decreases but the percentage by weight of the species having a molecular weight greater than the molecular Weight having the greatest number of polymers increases and many species have a molecular Weight well above the average.

These normal distributions are found in the casev of high polymeric substances such as natural rubber, polybutene (Vistanex), pol ystrene and copolymers of butene and isoprene (butyl rubber), and of butadiene and styrene (Buna S).

According to the extent of reaction allowed, the properties of these polymeric substances, such as toughness, tensile strength, viscosity, and others, can be altered within limits, such properties varying continuously with the change in normal distribution which isa mathematical function of the extent of reaction. However the variation does not approach the amount of variation in properties that can be secured by the utilization of this invention, which is founded upon the discovery that elasto-viscous materials comprising pol ymolecular systems of hydrocarbon polymers can beformed with stable or stabilized molecular weight distributions so much at variance with the normal molecular weight distribution of unperturbed polymeric systems as to yield distinctive products which, in and of themselvesor merely with the inclusion of inert fillers-are practicably useful as pressure-sensitive adhesives, in contradistinction to the normal distribution systems, such as rubber, polybutene or Buna S, which require modification with one or more molecularly foreign modifiers before they are thus practicably useful. In practice, the variation is usually such that the system is characterized by a materially increased concentration of polymers of molecular weight ranging about a relatively low molecular weight, over the normal concentration of such polymers in a normal polymeric distribution. v

A main resulting advantage is that the adhesive has substantially the electrical properties of the monomerically related or homogeneous hydrocarbon elasto-viscous material, unimpaired by the tion or functions in contrast to such abnormal,

unnatural or perturbed distribution function or functions.

The normal polymeric distribution in polymeric substances of the type useful in the formation of adhesives of this invention is of the type determined by Flory for linear polymers (J. A. C. S. 58, 1877 (1936) according to the equation m=wp (lp) 2 where m =the proportion by weight of polymers of chain length as x=the chain length; and

p=the extent of polymerization reaction For present purposes, this equation can be generallized in the expression representing normal molecular weight distribution in a polymeric substance formed by any natural or synthetic polymerization process, as a function (g) of chain length (x) and extent of reaction (p).

With this generalization, the molecular Weight distributions of adhesives of this invention are identified by the mathematical expressions,

(111) wherein m=u1 proportion by weight of polymers of chain length ":c; 1

az=the chain length;

101, p2 p are the extents of reaction in polymeric substances 1, 2 n; W1, W2, W11. are the weights of polymeric substances in defferent ranges (1, 2 n) of molecular weights: and

91 (III, P1), 92 (56, p2) Q11. ((17, 2311) An abnormal or unnatural distribution con-- forming to expression (I) is illustrated by a nor-- mal distribution system which has been perturbed as by a fractionation technique to remove or disproportionately reduce certain selected components or groups of components of particular mo lecular weight or Weights.

An abnormal or unnatural distribution con-- forming to expression (II) is illustrated by a polymeric system composed of a combination of two or more normal distribution systems, such as a low molecular weight system or a monomeric system with a higher molecular weight system or systems.

An abnormal or unnatural distribution conforming to expression (III) is illustrated by a polymeric system composed of a combination oi one or more abnormal distribution systems with one or more abnormal or normal distribution systems.

The perturbation from normal distribution of elasto-viscous materials of this invention is, in most cases, so marked that, graphically, the number of polymer molecules plotted against particular molecular weight will produce a curve havin two or more definite maxima indicating concentration of polymers around at least two different particular molecular weights, usually Widely separated; or by a curve characterized by a flattened segment or pronounced platy-allokurtosis; in contradistinction to normal distribution curves indicating a definite maximum concentration of polymers around a single molecular weight.

The elasto-viscous materials contemplated by this invention comprise predeterminedly controlled perturbed stable or stabilized polymeric systems, as above described, consisting of ethylenic polymerization or copolymerization products of olefins, such as ethylene and butane, diolefins, such as butadiene and isoprene, and monoderivatives of olefins and diolefins, such as styrene and chloroprene, the normal distribution systems whereof are not, as above indicated, per se useful as commercially satisfactory pressure-sensi-- tive adhesives.

Adhesive tape fabricated in accordance with this invention with certain of such materials is claimed in my copending application Serial No."

5,; 536,198, filed May 18, 1944', which, in turn, was a continuation-in-part of my now" abandoned allowed application Serial- No. 380,013; filed F'ebruary 21, I941.

This application, which is a division of said copending' application; isdirected to certain adhesive tapes formulatedwith polymeric andm'onomeric styrene:

Polystyrene, a polymer of vinyl benzene; is a further example of an elasto-viscous material which may be prepared in fractions of widely separated molecular weight and combined to form useful pressure-sensitive adhesives.

I havev discovered. that a polystyrene adhesive may be prepared by partially polymerizing a quantity of monomeric styrene, and then dissolving the partially polymerized composition, which is at a predeterminedly controlled degree of polymerization, in afurther quantity of styreneeither monomeric styrene or polymeric styrene at a lower degree of polymerization, or mixtures thereofand which second quantity of Styrene contains a styrene polymerization inhibitor or retardin agent. Such ingredients maybe formulated so that the composition has. suitable viscosity for spreading on a. backing and so that further polymerization will: be definitely controlled with the. adhesive retaining sticky, tacky characteristics over substantial periods of; time after spreading, by proper packaging, whereby, upon removal from the'pac'kage, the coating. Will befresli, soft and tacky; as is required for classificat'ion as a pressure-sensitive adhesive, and will; remain so at room temperatures for a period sufficiently long to permit manipulation of the coated. backing as a flexible. pressurersensitive adhesive tape with application in; the customary manner to electrical or other parts, to which the tape Willconform' by reason ofits. flexibility.

The following is an example of'a typical preparation:

Seventy-two parts of styrene monomer are polymerizedat 125 C. for aperiod of. approximately 3 hours, then cooled as rapidly as possible and mixed with 22 parts of styrene monomer. Since styrene monomer is unstable, the above operations may be advantageously conducted in a nitrogen atmosphere. This composition has a viscosity suitable for doctor spreading on any suitable sheet backing, for instance, of glass, polystyrene r polybutene. In order however 'to maintain stability, it is advantageous to incorporate in the mix .05% of a styrene polymerization inhibitor in the form of hydroquinone, or equivalent amounts of quinone, benzoquinone or phenanthanquinone.

The composition as applied will have soft, sticky, tacky characteristics and will be transparent. Where the coating is to be thin, the spreading operation is also advantageously conducted in the absence of oxygen. Application of the coating may be to one or both sides of the backing, or by an impregnating operation, the supporting medium, if of the porous type like woven glass fabric, can be provided with a pressure-sensitive tacky adhesive on both sides. Whether the operation be one of coating or of impregnating, a thin continuous flexible film of the composition may be secured. In some cases volatile hydrocarbon solvents may be resorted to as thinners or diluents for the purpose of spreading the compositions on the backing.

Again ground or pulverized solid polymeric styrene or any one of the several hard varieties of polystyrene, finely ground mica, ground silica G. (quartz), ground glass,- may, be included as inert fillers Because of the unstable characteristics ofstyrene polymers, especially thoseof low molecular weight; packaging precautions are required, not found in the" case of polybutene and other ofthe polymeric adhesives described in thisapplication. In the c'ase of polystyrene adhesive, it is sunicient for some conditions to package the" tape' in rolled form; with or without an interliner, in an airtlght, light tight container; Under other conditions, it is advantageous toevacuate partially the container of air or to fill the container with a gaswhich will not catalyze styrene polymerization, examplesof such gases being nitrogenand carbon dioxide. Whe're'qua-litieg permitting storage for maximum time periods are required, instead of using a gaswh-ich merel is inactive so far as the polymerization reaction is concerned; it maybe found desirable to introduce a gas of a type which of itself tends to inhibit styrene polymerization. The gaseous hyd'roca-rbonsisobutane; butyl'ene' and propane are gases useful for this purpose;

The stability of the tape may also be controlled depending upon the" particular" type and amount of inhibitor included. In the usual case, the polymerization retarding effect of the inhibitor is overcome in the course ofexposure to oxygen over a period of: time with resulting gradual polymerization of' the styrene adhesive into a dry mass. Such final setting'up of the adhesive to form in some cases a bond between the backing and the article to which the tape has been applied, may'ii desired be accelerated byapplication of heat and/or bythe inclusion in the adhesive' mass of oxygen catalysts, preferably those becoming active at elevated temperature.

The particular properties of all the polymeric hydrocarbon adhesive. masses of this invention, and especially their viscosity and tack properties, may be established through suitable control of certain variables. The first of these is the particular character'ofthe unnatural distribution or degree of perturbation; depending'upon the particular' fractions used, which in turn depend, in the case of fractionation preparation, upon the particular precipitate secured or, in the case of incomplete polymerization preparation of the fraction from the monomer or from lower polymers, upon the degree of polymerization resulting from the particular temperature and duration and/or catalyst of polymerization. Where the monomer is a liquid, as in the case of styrene, the monomer may be used as a unique but usable molecular weight species for varying the particular molecular weight distribution and thus the particular characteristics of the elasto-viscous material. In general, as above illustrated, adhesives of general use are obtained by utilizing as a sole tack-inducing or plasticizing ingredient for a high molecular weight polymer, polymers of low molecular weight or weights, preferably having a viscosity like those above given, of the order of 10 -10 poises-as distinguished from solid tack-inducing plasticizers such as resins having at room temperature high viscosities of the order of 10 poises-to produce elasto-viscous materials having at room temperature viscosities running from about 10 poises down to 10 poises; and such low molecular weight plasticizers, as above indicated, may be monomerically identical with the high polymer. A natural high polymer of natural unperturbed molecular weight distribution such as natural rubber has or" combinationsthereof a viscosity at room temperature of 10" running to perhaps 10 poises.

Secondly, the viscosity and consistency of the mass may be controlled by properly proportioning the amount of individual fractions which are .interadmixed in the formation of the adhesive.

The consistency may also be modified in the direction of greater viscosity by resorting to a vacuum distillation of one or more of the fractions utilized or of the final adhesive, thereby removing a portion of monomer which may remain in the fractions or the adhesive.

Where styrene is used in an intermediate or incomplete stage of polymerization, it will be understood that inclusion of the inhibitor is essential for preservation of the adhesive in a stable condition.

The term stable polymeric hydrocarbon elasto-viscous material as used herein connotes a polymeric hydrocarbon elasto-viscous material which is stable against further polymerization either in and of itself or by reason of the presence of a polymerization retarding agent or inhibitor in minute quantities, for instance, of the order of 05% by Weight of the hydrocarbon ingredient oi the elasto-viscous material.

I claim:

, 1. Fibrous tape impregnated with a mixture containing'a polymerized aromatic vinyl hydrocarbon and a monomeric polymerizable aromatic hydrocarbon, said monomeric hydrocarbon being present in a proportion between about 10% and about 50% of said polymerized hydrocarbon, producing a sticky tape that hardens upon polymerization of said monomeric hydrocarbon.

, 2. Fibrous tape impregnated with a mixture containing polystyrene and monomeric styrene, said styrene being present in a proportion between about 20% and about 50% of said polystyrene, producing a sticky tape that hardens uponpolymerization of said monomeric styrene. 3, An adhesive tape comprising a sheet backing coated with a mixture containing polystyrene and monomeric styrene, said monomeric styrene being present in a proportion between about 10% and about 50% of said polystyrene, producing a sticky REFERENCES CITED The following references are of record in the file of this patent and in the parent case:

UNITED STATES PATENTS Number Name Date 1,842,456 MacKenzie Jan. 26, 1932 2,042,333 Coe May 26, 1936 2,051,840 Gerhart Aug. 25, 1936 2,061,570 Frolich Nov. 24, 1936 2,103,841 Calvert Dec. 28, 1937 2,106,850 McCulloch Feb. 1, 1938 2,124,234 Mueller-Cunradi et a1. July 19, 1938 2,145,350 Haslam Jan. 31, 1939 2,147,824 Webb Feb. 21, 1939 2,238,694 Graver Apr. 15, 1941 2,256,160 Britton et al Sept. 16, 1941 2,285,570 Cummins et a1 June 9, 1942 2,319,959 Tierney May 25, 1943 2,365,646 New et al Dec. 19, 1944 2,395,419 Mitchell Feb. 26, 1946 2,415,276 Buckley et a1 Feb. 4, 1947 2,421,640 New et a1 June 3, 1947 2,439,481 Martin Apr. 13, 1948 FOREIGN PATENTS Number Country Date 401,500 Great Britain Nov. 16, 1933 553,766 Great Britain 1943 

1. FIBROUS TAPE IMPREGNATED WITH A MIXTURE CONTAINING A POLYMERIZED AROMATIC VINYL HYDROCARBON AND A MONOMERIC POLYMERIZABLE AROMATIC HYDROCARBON, SAID MONOMERIC HYDROCARBON BEING PRESENT IN A PROPORTION BETWEEN ABOUT 10% AND ABOUT 50% OF SAID POLYMERIZED HYDROCARBON, PRODUCING A STICKY TAPE THAT HARDENS UPON POLYMERIZATION OF SAID MONOMERIC HYDROCARBON. 